Jalkut sandboxing fixes

Hey guys - here are some tweaks for your consideration that I think make the build process more robust without causing any harm to the way you may be using implicit dependencies.

The big win here I think is the use of an iMedia.stub file for linking the XPC targets, so we don't have a dependency cycle in the building of these. It's now always possible for XPC targets to be built before the iMedia target as been built even once.